We have completed a study comparing different inhibitors including benzylpyrimindines and triazines with many different species of dihydrofolate reductase. This study included enzyme assays in in vivo testing. By comparing Quantitative Structure-Activity Relations with molecular models, we have been able to gain valuable insight into species specificity. We are currently expanding this work to on dihydrofolate reductase to P. Carnii, the causative agent of pneumonia in AIDS patients.